feat(elements): make ElementDeclaration fields nullable

Optional opt-in via field presence — declaring a field is enough.
Drops `HasImage`/`HasShadow`/`HasBorder`/`HasCustomData`/`IsFloating`/
`IsScrollable` sentinel checks in ConfigureOpenElement; checks for
`is not null` instead. Layout falls back to default when null. Id
stays required.

ClayUI widgets that previously assigned `Shadow = s.Shadow` /
`Border = s.Border` unconditionally now guard with `HasShadow` /
`HasBorder` so empty styles map to null. Avoids spurious Shadow
render commands (the renderer emits unconditionally on config
presence) and saves storage for empty Border configs.

Adds OptionalElementDeclarationTests (23 cases) covering null-by-
default, declaration-is-enough (empty Custom/Shadow/Scroll), and
regression for factory methods + existing styled paths.

Author: andreakarasho

src/Clay.Test/ConfigTests.cs

@@ -206,21 +206,21 @@ public void Container_SetsLayout()
     {
         var layout = LayoutConfig.FillRow(gap: 8);
         var decl = ElementDeclaration.Container(layout);
-        Assert.Equal(8, decl.Layout.ChildGap);
+        Assert.Equal(8, decl.Layout!.Value.ChildGap);
     }
 
     [Fact]
     public void Box_SetsLayoutAndColor()
     {
         var decl = ElementDeclaration.Box(LayoutConfig.FillRow(), Color.Red);
-        Assert.Equal(Color.Red.R, decl.BackgroundColor.R);
+        Assert.Equal(Color.Red.R, decl.BackgroundColor!.Value.R);
     }
 
     [Fact]
     public void RoundedBox_SetsCornerRadius()
     {
         var decl = ElementDeclaration.RoundedBox(LayoutConfig.FillRow(), Color.Red, 8f);
-        Assert.True(decl.CornerRadius.HasRadius);
+        Assert.True(decl.CornerRadius!.Value.HasRadius);
     }
 }